The present invention generally concerns elemental silicon nanoparticles.
Silicon nanoparticles are an area of intense study. Nanoparticles that are below about 10 nm are of interest because the nanoparticles, when reduced to particular sizes not found in naturally occurring elemental silicon, exhibit properties unlike bulk silicon. Various techniques exist for producing silicon nanoparticles of random dimensions, and those processes sometimes produce silicon nanoparticles below 10 nm.
U.S. patent application Ser. No. 09/426,389, entitled SILICON NANOPARTICLE AND METHOD FOR PRODUCING THE SAME, incorporated by reference herein, discloses a method for producing quantities of uniformly sized 1 nm silicon nanoparticles. This ability to produce uniformly sized particles in quantity is an important advance in the state of the art.
The invention is a family of discretely and uniformly sized silicon nanoparticles, including 1 (blue emitting), 1.67 (green emitting), 2.15 (yellow emitting), 2.9 (red emitting) and 3.7 nm (infrared emitting) nanoparticles, and a method that produces the family. The nanoparticles produced by the method of the invention are highly uniform in size. A very small percentage of significantly larger particles are produced, and such larger particles are easily filtered out.
The method for producing the silicon nanoparticles of the invention utilizes a gradual advancing electrochemical etch of bulk silicon, e.g., a silicon wafer. The etch is conducted with use of an appropriate intermediate or low etch current density. An optimal current density for producing the family is xcx9c10 milli Ampere per square centimeter (10 mA/cm2). Higher current density favors 1 nm particles, and lower the larger particles. Blue (1 nm) particles, if any appreciable quantity exist depending on the selected current density, may be removed by, for example, shaking or ultrasound. After the etch, the pulverized wafer is immersed in dilute HF for a short time, while the particles are still connected to the wafer, to weaken the linkages between the larger particles. This may be followed by separation of nanoparticles from the surface of the silicon. Once separated, various methods may be employed to form plural nanoparticles into crystals, films and other desirable forms. The nanoparticles may also be coated or doped. The invention produces the family of a discrete set of sized particles and not a continuous size distribution.
Particles may be isolated from the family, i.e., it is possible to produce any one of the sizes of particles from the family after the basic method steps have been executed to produce the family of particles. The blue particles are preferably obtained prior to the HF immersion. Larger particles are obtained by additional procedures subsequent to the HF immersion. An isolation step is conducted to select a desired particle size from the family of particles after the above steps to produce the family of particle are conducted. According to a preferred embodiment, isolation is accomplished using steps of centrifuging, course filtering, and acid treatment. This protocol is effective in isolating particles of a given size with a high degree of uniformity. Chromatography may be used to further isolate particles of a given size with a high degree of uniformity since the sizes are discrete and well separated.